Alkylene oxides such as ethylene oxide, propylene oxide and 1,2-butylene oxide are polymerized to form a wide variety of polyether products. For example, polyether polyols are prepared in large quantities for polyurethane applications. Other polyethers are used as lubricants, brake fluids, compressor fluids, and many other applications.
These polyethers are commonly prepared by polymerizing one or more alkylene oxides in the presence of an initiator compound and an alkali metal catalyst. The initiator compound is typically a material having one or more hydroxyl, primary or secondary amine, carboxyl or thiol groups. The function of the initiator is to set the nominal functionality (number of hydroxyl groups/molecule) of the product polyether, and in some instances to impart some desired functional group to the product.
Until recently, the catalyst of choice was an alkali metal hydroxide such as potassium hydroxide. Potassium hydroxide has the advantages of being inexpensive, adaptable to the polymerization of various alkylene oxides, and easily recoverable from the product polyether.
However, to a varying degree, alkali metal hydroxides catalyze an isomerization of propylene oxide to form allyl alcohol. Allyl alcohol acts as a monofunctional initiator during the polymerization of propylene oxide. Thus, when potassium hydroxide is used to catalyze propylene oxide polymerizations, the product contains allyl alcohol-initiated, monofunctional impurities. As the molecular weight of the product polyether increases, the isomerization reaction becomes more prevalent. Consequently, poly(propylene oxide) products prepared using KOH as the catalyst at equivalent weights of about 800 or more tend to contain very significant quantities of the monofunctional impurities. This tends to reduce the average functionality and broaden the molecular weight distribution of the product.
More recently, the so-called double metal cyanide (DMC) catalysts have been used commercially as polymerization catalysts for alkylene oxides. These DMC catalysts are described, for example, in U.S. Pat. Nos. 3,278,457, 3,278,458, 3,278,459, 3,404,109, 3,427,256, 3,427,334, 3,427,335 and 5,470,813, among many others. Because these catalysts do not significantly promote the isomerization of propylene oxide, polyethers having low unsaturation values and higher molecular weights can be prepared, compared to potassium hydroxide-catalyzed polymerizations.
Unfortunately, the DMC catalysts have other significant drawbacks. DMC catalysts are difficult to separate from a polyether polyol. As a result, most of the time the catalyst is simply left in the polyol. This requires that the catalyst be continually replaced, which adds to the cost of producing the polyether. In some cases, the DMC catalyst interferes with downstream uses of the polyol, and so cannot be left in the polyol. Perhaps more important is that DMC catalysts are not effective in producing ethylene oxide-capped poly(propylene oxides). These capped polyols represent a significant portion of the demand for polyols for polyurethane applications. As a result, polyol manufacturers using DMC catalysts must in addition conduct separate ethylene oxide-capping processes using in most cases conventional alkali metal hydroxide catalysts.
Even more recently, certain phosphazene and phosphazenium compounds have been mentioned as alkylene oxide polymerization catalysts. See, for example, U.S. Pat. Nos. 5,952,457 and 5,990,352, as well as EPO-A-0 763 555, 0 879 838, 0 897 940, 0 916 686 and 0 950 679. These compounds are reported to provide good polymerization rates and to provide poly(propylene oxide) polymers having low levels of unsaturation. Moreover, these compounds are capable of producing ethylene oxide-capped poly(propylene oxides). However they are quite expensive and difficult to separate from the product polyether. Thus, the polyol manufacturer must either engage in expensive steps to recover the catalyst, or else ship the product with the catalyst still in it. Either option significantly increases the cost of the polyether. Moreover, the strongly basic catalyst interferes with many downstream uses of the polyether.
It would be desirable to provide an alkylene oxide polymerization catalyst that provides good polymerization rates, produces poly(propylene oxide) polymers with low levels of unsaturation, allows for the production of ethylene oxide-capped poly(propylene oxide) polymers, and is inexpensively removed from the product polyol.
In one aspect, this invention is a cross-linked organic polymer having pendant phosphazene groups including at least two phosphorus atoms or phosphazenium groups including one or more phosphorus atoms.
In another aspect, this invention is a method comprising subjecting an alkylene oxide to polymerization conditions in the presence of an initiator compound and a catalytically effective amount of a crosslinked organic polymer having pendant phosphazene or phosphazenium groups, wherein said crosslinked organic polymer is substantially insoluble in said alkylene oxide and said polyether.
In this invention, a crosslinked organic polymer having pendant phosphazene or phosphazenium groups is used as a catalyst for an alkylene oxide polymerization.
By xe2x80x9cphosphazenexe2x80x9d group, it is meant an uncharged group containing a chain of alternating nitrogen and phosphorus atoms which contains at least two nitrogen atoms in the chain. The phosphazene group will contain at least one xe2x80x94Nxe2x95x90Pxe2x80x94Nxe2x80x94 linkage in the chain. It is preferred that the phosphazene group contains at least two phosphorus atoms in the chain. The phosphazene group more preferably has from about 2 to about 6 phosphorus atoms. The chain of nitrogen and phosphorus atoms may be branched. It is most preferred that each phosphorus atom is bound to four nitrogen atoms. Typically, each phosphorus atom will be singly bonded to three nitrogen atoms, and doubly bonded to a fourth nitrogen atom.
A xe2x80x9cphosphazeniumxe2x80x9d group is a corresponding cationic group. With phosphazenium groups, it is preferred that the chain contains from about 1 to about 6 phosphorus atoms. As before, the chain of nitrogen and phosphorus atoms may be branched. It is most preferred that each phosphorus atom is bound to four nitrogen atoms.
Thus, polymers containing phosphazene groups can be represented by the structures
Polymer-[Nxe2x95x90P{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3]zxe2x80x83xe2x80x83(I) 
and
Polymer-{NR1xe2x80x94[P(A2)xe2x95x90N]xxe2x80x94P(A2)xe2x95x90NR}zxe2x80x83xe2x80x83(IA) 
Similarly, polymers containing phosphazenium groups can be represented by the structures
Polymer-[NR1xe2x80x94P+{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3]zxe2x80x83xe2x80x83(II) 
and
Polymer-{NRxe2x80x94[P(A2)xe2x95x90N]xxe2x80x94P+(A2)xe2x80x94NRR1}zxe2x80x83xe2x80x83(IIA) 
In these formulae, each R and R1 is independently in each occurrence (a) an unsubstituted or inertly substituted alkyl or aryl group, (b) an unsubstituted or inertly substituted alkylene or arylene group that, together with another R or R1 group on the same nitrogen atom, forms a ring structure including that nitrogen atom, (c) an unsubstituted or inertly substituted alkylene or arylene group that, together with a R or R1 group bonded to a different nitrogen atom bonded to a common phosphorus atom, forms a ring structure including an xe2x80x94Nxe2x80x94Pxe2x80x94Nxe2x80x94 or xe2x80x94Nxe2x80x94Pxe2x95x90Nxe2x80x94 moiety, or (d) hydrogen. Each R is preferably a C1-10 alkyl group, or together with another R forms a C2-5 alkylene group that forms part of a ring structure with a nitrogen atom or an xe2x80x94Nxe2x80x94Pxe2x80x94Nxe2x80x94 or xe2x80x94Nxe2x80x94Pxe2x95x90Nxe2x80x94 moiety. Each R1 is preferably hydrogen, methyl, ethyl, n-propyl or isopropyl, or together with another R forms a C2-5 alkylene group that forms part of a ring structure with a nitrogen atom or an xe2x80x94Nxe2x80x94Pxe2x80x94Nxe2x80x94 or xe2x80x94Nxe2x80x94Pxe2x95x90Nxe2x80x94 moiety. Each A is independently xe2x80x94[Nxe2x95x90P(A2)]xxe2x80x94NR2, where R is as before. Each x is independently zero or a positive integer. The values of the various x""s are preferably such that the group contains 2-6 phosphorus atoms in the case of phosphazene groups, and 1-6 phosphorus atoms in the case of phosphazenium groups. z is a positive integer.
It is noted that the charge on any phosphazenium group will be delocalized throughout the phosphazenium group and may not reside on the particular phosphorus atom shown as carrying the charge in formulae II or IIA. Although not shown in formulae II or IIA, the phosphazenium group in all cases will be associated with a counterion, which can be, for example, halogen, hydroxide, nitrate, sulfate, and the like. The counterion can also be an alcoholate. Preferred counterions are hydroxyl and alcoholates formed by the extraction of an alcoholic proton from a polyhydric initiator compound. It has been found that the catalyst has greater activity when the counterion on the phosphazenium groups is hydroxyl or alcoholate. The groups can be converted to the hydroxyl form by washing the catalyst with a solution of an alkali metal hydroxide such as sodium hydroxide in water or a mixture of water and methanol. At least a stoichiometric amount of hydroxide ions are provided based on the phosphazenium groups.
The crosslinked organic polymer is any polymer that is (1) sufficiently cross-linked to be insoluble in alkylene oxides and polyether polyols made from the alkylene oxides, (2) is chemically stable under strongly basic conditions, and (3) does not react in an undesirable way with a phosphazene or phosphazenium group, an alkylene oxide or a polyether. They are preferably particulate and have a particle size such that they can be separated from a liquid by filtration. Particle sizes of from about 0.1 mm to about 5 mm are especially suitable. A preferred, commonly available type of crosslinked organic polymer is a crosslinked alkenyl aromatic polymer, particularly a crosslinked polymer or copolymer of styrene. Especially preferred crosslinked organic polymers are particulate, swellable copolymers of styrene and a crosslinking agent such as divinyl benzene. Commercially available absorbent resins of this type are suitable. So-called macroporous types as well as the so-called gel-type resins are useful crosslinked polymers. Such copolymer resin beads are commercially available from Aldrich Company.
Because of the chemistry of phosphazene and phosphazenium groups, it is convenient to use an organic polymer that contains halogen, especially aliphatic halogen, or primary or secondary amine groups as a starting material. Halogenation can be introduced into the organic polymer by, for example, polymerizing or copolymerizing a halogenated monomer into the polymer. Halogenated crosslinked alkenyl aromatic polymers are conveniently prepared through a haloalkylation reaction, such as by the well-known reaction of the polymer with chloromethyl methyl ether. These reactions introduce haloalkyl groups onto the aromatic rings of the polymer. Amine-functional organic polymers are conveniently made by reacting a halogenated organic polymer with ammonia or a primary amine.
Phosphazene and phosphazenium groups can be attached to the organic polymer support in various ways. In general, the two main methods are (1) to attach a previously-formed phosphazene or phosphazenium group to the organic polymer, and (2) to xe2x80x9cbuildxe2x80x9d the phosphazene or phosphazenium group onto the organic polymer. Note that in many cases, once a phosphazene group is attached to an organic polymer support, it can be converted to a phosphazenium group. This can be done, for example, by reacting the phosphazene group with an alkyl halide having the structure R1X, where X denotes the halide ion, especially a chloride, bromide or iodide ion, to form the halide salt of the corresponding phosphazenium. This reaction is conveniently performed in an inert solvent such as tetrahydrofuran and at an elevated temperature such as about 40-100xc2x0 C. Conversely, those phosphazenium groups having an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage can be converted to the corresponding phosphazene by reaction with a suitable strong base, as described more below.
Several preparation methods are described below. Methods A and B involve attaching a previously formed phosphazenium group onto the polymer. Methods C-G involve xe2x80x9cbuildingxe2x80x9d the groups on the polymer.
1. Method Axe2x80x94Direct Preparation of Phosphazenium Groups
The phosphazenium-substituted polymer can be prepared by introducing halogen, preferably haloalkyl substitution, onto the crosslinked polymer. The halogen-substituted polymer is then reacted with a phosphazene compound of the structure
NR1xe2x95x90P{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3xe2x80x83xe2x80x83(III) 
The halogen salt of the phosphazenium-substituted polymer is formed directly and has a structure corresponding to II above. It has been found that this reaction goes very slowly in most cases, except when R1 is hydrogen, ethyl, methyl, propyl and isopropyl. When the resulting phosphazenium groups contain an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage, the corresponding phosphazene-substituted polymer can be prepared by treating the phosphazenium-substituted polymer with a strong base such as an alkali metal hydride (e.g., potassium hydride) or an alkali metal hydroxide (e.g., potassium or cesium hydroxide), sodium amide, another phosphazene compound or the like. In general, the larger the phosphazenium group, the stronger the base that is required to convert it to the phosphazene.
2. Method Bxe2x80x94Direct Preparation of Phosphazenium Groups
A second way of making the phosphazenium-substituted polymer is to react a halogen-substituted polymer with a phosphazenium compound of the structure
[NHR1xe2x80x94P+{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3]Xxe2x80x94xe2x80x83xe2x80x83(IV) 
in the presence of a strong base. Again, a phosphazenium-substituted polymer corresponding to structure II is formed directly. When the phosphazenium groups contain an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage, the corresponding phosphazene-substituted polymer can be prepared by treating the phosphazenium-substituted polymer with a strong base as described before. An example of a reaction sequence of this type is: 
3. Method Cxe2x80x94xe2x80x9cBuildingxe2x80x9d Phosphazenium Groups Directly Onto a Polymer with Pendant xe2x80x94NH2 groups.
In this method, an amine-substituted organic polymer is reacted with phosphorus pentachloride and then with an excess of a compound having the structure NHxe2x95x90P{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3. The corresponding phosphazenium group (in the chloride form) is formed directly. It is preferred to convert the resin to the hydroxide or alcoholate form as discussed before. When the phosphazenium groups contain an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage, they may be converted to phosphazene groups by treatment with a strong base as described before. An example of a synthesis of this type is shown below: 
4. Method Dxe2x80x94xe2x80x9cBuildingxe2x80x9d Phosphazenium Groups Directly Onto Certain Diamine-Substituted Polymers
A variation of the technique just described uses a diamine-substituted organic polymer as a starting material. The amine groups each have at least one amine hydrogen atom and are separated by one or more, preferably about 3, methylene groups. This starting material is reacted with phosphorus pentachloride, then with an excess of a compound having the structure NHxe2x95x90P{[xe2x80x94Nxe2x95x90P(A2)]xxe2x80x94NR2}3. The resulting phosphazenium groups in the chloride form have a cyclic structure including a xe2x80x94Nxe2x80x94Pxe2x95x90Nxe2x80x94 moiety. It is preferred to convert the resin to the hydroxide or alcoholate form as discussed before. When the phosphazenium groups contain an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage, they may be converted to phosphazene groups by treatment with a strong base as described before. An example of such a synthesis follows: 
5. Method Exe2x80x94xe2x80x9cBuildingxe2x80x9d a Phosphazenium Group Using [Cl3P+xe2x80x94Nxe2x95x90PCl3]PCl6xe2x80x94
In yet another synthesis technique, an organic polymer having pendant xe2x80x94NH2 groups is reacted with [Cl3P+xe2x80x94Nxe2x95x90PCl3]PCl6xe2x80x94 and then with an excess of a secondary amine compound having the structure NHR2. Phosphazenium groups in the chloride form are prepared. The chloride groups can be exchanged for hydroxyl or alcoholate groups as described before. Also, when the phosphazenium groups contain an xe2x80x94NHxe2x80x94Pxe2x80x94 linkage, the corresponding phosphazene groups can be formed by treatment with a strong base, as before. An example of this synthesis is shown below: 
5. Method Fxe2x80x94xe2x80x9cBuildingxe2x80x9d a Phosphazenium Group Using [Cl3P+xe2x80x94Nxe2x95x90PCl3]PCl6xe2x80x94 and an xe2x80x94NHR-substituted Polymer
In a variation of the technique just described, the organic polymer contains pendant xe2x80x94NHR groups instead of xe2x80x94NH2 groups. In this case, the phosphazenium group is formed, but cannot be easily converted to the corresponding phosphazene. An example of this synthesis is shown below: 
7. Method Gxe2x80x94xe2x80x9cBuildingxe2x80x9d Phosphazene Groups
A polymer containing pendant xe2x80x94NHR groups is reacted with a bis(disubstituted amino) phosphorus oxychloride ((R2N)2POCl) to form pendant xe2x80x94NRxe2x80x94P(O)xe2x80x94(NR2)2 groups. These groups are then chlorinated by reaction with a chlorinating agent such as phosphorus oxychloride (POCl3), chlorine, and the like, and reacted with ammonia and a base to form pendant xe2x80x94NRxe2x80x94P(NR2)2xe2x95x90NH groups. To form terminal xe2x95x90NR groups, a primary amine of the form H2NR is used instead of ammonia. The sequence of reactions with bis(disubstituted amino) phosphorus oxychloride, phosphorus oxychloride, ammonia (or primary amine) and base may be repeated one or more times. In any of these reactions, the bis (disubstituted amino) phosphorus oxychloride may be replaced with an A2POCl compound to introduce branching. The phosphazene group prepared in this way can be quaternized by reaction with an organic halide (R1X), as described before.
Phosphazenes according to structures (III) and (IV) above can be prepared according to processes such as are described in EP-A-0 879-838, particular pages 6-7 thereof, incorporated herein by reference. In addition, the synthesis of phosphazene and phosphazenium compounds is described in Schwesinger, R.; et. al. Liebigs Ann. 1996, 1055-1081, Schwesinger, R.; et. al. Chem. Ber. 1994, 127, 2435-2454, Schwesinger, R. Nachr. Chem. Tech. Lab. 1990, 38(10), 1214-1226, Schwesinger, R.; Schlemper, H. Angew. Chem. Int. Ed. Engl. 1987, 26(11), 1167-1169 and Schwesinger, R.; et. al. Angew. Chem. Int. Ed. Engl. 1993, 32(9), 1361-1363. Techniques described therein can be used to make starting materials for use herein, and can be adapted for making the phosphazene or phosphazenium-substituted polymer.
The product is a supported catalyst that may have, for example, from about 0.1 to about 10 mmols of phosphazenium groups per gram of supported catalyst.
It is preferred to wash the catalyst with water and/or methanol or similar organic solvent, and then to dry it well, such as in a vacuum oven, before use in order to remove impurities and undesired by-products of the synthesis reactions.
The supported catalyst of the invention is used to polymerize alkylene oxides to make polyethers. In general, the process includes mixing a catalytically effective amount of the catalyst with an alkylene oxide under polymerization conditions and allowing the polymerization to proceed until the supply of alkylene oxide is essentially exhausted. The concentration of the catalyst is selected to polymerize the alkylene oxide at a desired rate or within a desired period of time. Enough of the supported catalyst is used to provide at least about 0.0001 millimole of phosphazenium groups per gram of polyether to be produced. As the supported catalyst is easily recovered from the product, any larger amount can be used as long as the reaction can be controlled. More preferred catalyst levels are from about 0.0005, most preferably from about 0.001, to about 0.1, most preferably to about 0.025 mmol of phosphazenium groups per gram of product polyether. In this context, the weight of the product polyether is considered to be equal to the combined weight of the initiator compound, if any, plus added alkylene oxides.
For making high molecular weight monofunctional polyethers, it is not necessary to include an initiator compound. However, to control molecular weight, impart a desired functionality (number of oxyalkylatable groups/molecule) or a desired terminal functional group, an initiator compound as described before is preferably mixed with the catalyst complex at the beginning of the reaction. The initiator compound contains one or more functional groups that is capable of being oxyalkylated with an alkylene oxide or oxetane, such as hydroxyl, primary or secondary amine, thiol and carboxylic acid groups. Suitable initiator compounds include monoalcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, t-butanol, octanol, octadecanol, 3-butene-1-ol, 2-methyl-2-propanol, 2-methyl-3-butene-2-ol and the like. Suitable polyalcohol initiators include ethylene glycol, propylene glycol, glycerine, 1,1,1-trimethylol propane, 1,1,1-trimethylol ethane, 1,2,3-trihydroxybutane, pentaerythritol, xylitol, arabitol, mannitol, sucrose, sorbitol, alkyl glucosides such as methyl glucoside and ethyl glucoside and the like. Polyether polyols are also useful initiator compounds. The suitable polyether polyols include those having a relatively low equivalent weight, such as less than 350 and especially from about 125-250. However, higher equivalent weight polyols, such as those having equivalent weights of up to 6000 or more, particularly from about 350 to about 4000, are also useful. These polyether polyol initiators can be, for example, polymers of propylene oxide, butylene oxide, tetramethylene oxide, ethylene oxide, and the like. Other types of polymers that contain oxyalkylatable groups, such as hydroxy-functional, thiol-functional, primary or secondary amine-functional polymers, (including functionalized polyethylenes, polystyrenes, polyesters, polyamides and the like) are suitable initiators.
Among the alkylene oxides that can be polymerized with the catalyst complex of the invention are ethylene oxide, propylene oxide, 1,2-butylene oxide, styrene oxide and mixtures thereof. Various alkylene oxides can be polymerized sequentially to make block copolymers. More preferably, the alkylene oxide is propylene oxide or a mixture of propylene oxide and ethylene oxide and/or butylene oxide. Especially preferred are propylene oxide alone or a mixture of at least 75 weight % propylene oxide and up to about 25 weight % ethylene oxide.
The method of the invention is suitably used to produce EO-capped polyols, especially to EO-cap polyols that have secondary hydroxyl groups such as poly(propylene oxide) and poly(butylene oxide) polyols. This can be done by (1) a sequential polymerization of PO or BO, followed by EO, using the supported phosphazenium or supported phosphazene catalysts of the invention or (2) by polymerizing EO (using the supported phosphazenium or supported phosphazene catalysts of the invention) onto a previously-formed poly(propylene oxide) polyol. The previously-formed poly(propylene oxide) polyol may be a homopolymer of propylene oxide or a random copolymer of PO, such as a random PO/EO copolymer. The previously-formed poly(propylene oxide) polyol may be formed using conventional alkali metal or alkaline earth metal hydroxide catalysts such as KOH, NaOH, CsOH, BaOH and the like, or using double metal cyanide (DMC) catalysts as are described in U.S. Pat. Nos. 3,278,457, 3,278,458, 3,278,459, 3,404,109, 3,427,256, 3,427,334, 3,427,335 and 5,470,813, among many others. In the case where the poly(propylene oxide) polyol is made using DMC catalysts, the DMC catalyst may be inactivated and/or removed prior to performing the EO capping, but this is not necessary. If desired, the DMC catalyst may be left in the poly(propylene oxide) polyol in an active form, until the EO capping step is done. The addition of the supported phosphazenium or phosphazene catalyst in the EO capping process will deactivate the DMC.
Especially preferred EO capped polyols made in this manner have an equivalent weight of about 1000-3000 and an EO cap that constitutes about 8-30% of the total weight of the polyol. However, the EO capping can be performed on lower equivalent weight polyol initiators (i.e., 125-1000 equivalent weight) to form EO caps that constitute, for example, up to 80% of the total weight of the polyol.
In addition, monomers that will copolymerize with the alkylene oxide in the presence of the catalyst complex can be used to prepare modified polyether polyols. Such comonomers include oxetanes as described in U.S. Pat. Nos. 3,278,457 and 3,404,109, and anhydrides as described in U.S. Pat. Nos. 5,145,883 and 3,538,043, which yield polyethers and polyester or polyetherester polyols, respectively. Hydroxyalkanoates such as lactic acid, 3-hydroxybutyrate, 3-hydroxyvalerate (and their dimers), lactones and carbon dioxide are examples of other suitable monomers that can be copolymerized with the catalyst of the invention.
The polymerization reaction typically proceeds well at temperatures from about 25 to about 150xc2x0 C. or more, preferably from about 80-130xc2x0 C. A convenient polymerization technique involves mixing the supported catalyst and initiator to form the phosphazenium alcoholate, and pressurizing the reactor with the alkylene oxide. Once the polymerization has begun additional alkylene oxide is conveniently fed to the reactor on demand, until enough alkylene oxide has been added to produce a polymer of the desired equivalent weight.
Another convenient polymerization technique is a continuous method. In such continuous processes, the initiator and alkylene oxide (plus any comonomers) are continuously fed into a continuous reactor, such as a continuously stirred tank reactor (CSTR) or a tubular reactor, that contains the supported catalyst of the invention. The product is continuously removed.
The catalyst is easily removed from the product polyether by filtration or other liquid/solid separation technique such as centrifugation. The recovered catalyst can be re-used in further polymerization reactions.
The recovered catalyst may be washed one or more times, preferably multiple times, with water or preferably an organic solvent such as methanol, and then dried prior to being re-used. If the surface of the catalyst becomes fouled or coated with polymer, the catalyst may be washed or treated to remove the fouling or polymer coating.